Предмет: Алгебра, автор: zhenya789

В арифметичекой прогрессии (an) : a1 = -3 ; a2 = -1 найдите сумму членов с 9-го по 21-й включительно. 

Ответы

Автор ответа: Аноним
0

d=-1-(-3)=2
a9=-3+2*8=13
a21=-3+2*20=37
s9-21=13+37/2*12=300
12 в этой действии это число членов с 9-го по 21 

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